In the manufacture of electrical components, such as transformer and inductor coils, it is a known practice to solder terminals of the coils to wire windings which have been wrapped on the terminals to form wire-wrapped connections, by dipping the terminals and the wire-wrapped connections in a molten solder fountain. The solder fountain is disposed in a molten solder bath and solder is pumped to the fountain by a pump comprising an impeller and an impeller housing submerged in the solder bath. The impeller is mounted on a lower end of a pump drive shaft usually journaled in a lower bearing in the impeller housing, and in an upper bearing located above the surface of the solder bath, with the shaft rotating directly in contact with an upper surface of the solder bath.
This arrangement, in which the pump shaft rotates directly in contact with the upper surface of the molten solder bath, is undesirable for various reasons. In this connection, a characteristic of molten solder is its tendency to burn in the atmosphere and form a residue known in the art as solder "dross", which normally floats on the upper surface of the solder bath. Thus, the solder fountain and the surface of the solder bath inherently form solder dross during the soldering operation, which dross must be periodically removed from the bath surface. When the pump shaft rotates in direct contact with the surface of the solder bath, this formation of dross is substantially increased as a result of the pump shaft agitating the solder bath and causing clean solder in the bath to be continuously circulated and exposed to the atmosphere. Further, it has been found that the dross tends to form in a cone-shaped configuration on and around the rotating pump shaft, with the dross eventually rising along the shaft into the upper shaft support bearing if not timely removed, causing the shaft to bind and freeze in the bearing. Minute dross particles which have become mixed into the solder bath by action of the rotating pump shaft and the flowing solder fountain also tend to work into the submerged lower shaft support bearing in the impeller housing, causing the shaft to freeze in this bearing. The same problem is still encountered when the pump shaft is supported in a bearing located at the surface of the solder bath so as to preclude agitation of the surface and the formation of dross by the rotating shaft. Similarly, even when the pump shaft has been enclosed and tightly packed with a packing material, such as a ceramic fiber blanket, in an attempt to isolate solder adjacent the shaft from the atmosphere, it has been found that the minute dross particles tend to work into the interface between the shaft and the packing material, creating a drag on the shaft and a reduction in its operating speed. As a result, prior known soldering apparatus must frequently be taken out of service for part maintenance and/or replacement purposes.
Accordingly, a primary purpose of this invention is to provide new and improved methods of and apparatus for pumping solder in which the formation of dross in a molten solder bath by surface agitation of the bath, and the adverse effects of dross on the solder pumping operation, are essentially eliminated.